A variety of (X-Y) type shifting mechanisms for shifting gears or clutches of a mechanical change gear transmission are known in the art. Such mechanical change gear transmissions characteristically feature a plurality of substantially parallel shift rails that are selectively engageable by pivoting a shift finger in a first (X--X) direction that is substantially transverse to the direction of their substantial parallel alignment and then moving the selected shift rail to effect the gear shift desired by moving the shift finger in a second direction (Y--Y) that is substantially transverse to the (X--X) direction and substantially parallel to the alignment direction of the shift rails.
An early example of a mechanical transmission fluid operated (X-Y) shifting mechanism is disclosed in U.S. Pat. No. 2,137,939, the disclosure of which is incorporated herein by reference. In this mechanism, shift finger (17) is pivoted by selector device (25) to selectively engage one of shift bars (22,23, or 24) but is secured to enlarged piston heads (4 and 15) as well as to enlarged piston heads (77 and 78) causing them all to rotate when shift finger (17) is pivoted by selector device (25) resulting in a substantial amount of rotational inertia and friction to be overcome in pivoting shift finger (17), particularly during cold weather operation when fluid seals and the like are in a hardened condition and the fluid itself more viscous.
Another example of a mechanical transmission fluid operated (X-Y) shifting mechanism is disclosed in U.S. Pat. No. 2,931,237, the disclosure of which is incorporated herein by reference. Here however, as shown in FIG. 3, shift rod actuator (211) is fixedly secured to shaft (192) which is received through and fixedly secured to piston (193) so that piston (193) is able to move shaft (192) axially but is also rotated in unison therewith when shaft (192) is rotated by actuator (211) requiring the seals in outer surface of piston (193) to rotate relative the large diameter bore surface and thereby creating high rotational inertia and friction to be overcome during shifting particularly during cold weather operating conditions. The mechanism described in U.S. Pat. No. 2,931,237 was known in the art as the "Fuelaire" system in which piston (193) was press fitted onto shaft (192) without seal (199).